1. Field of the Invention
This invention relates to a radiation image recording and read-out method for exposing a stimulable phosphor sheet carrying a radiation image stored therein to stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the radiation energy stored, and photoelectrically detecting the emitted light by use of a photodetector, and an apparatus for carrying out the method.
2. Description of the Prior Art
A novel radiation image recording and reproducing system is disclosed, for example, in U.S. Pat. Nos. 4,258,264, 4,276,473 and 4,315,318 and Japanese Unexamined Patent Publication No. 56(1981)-11395. The system comprises (i) exposing a stimulable phosphor sheet to a radiation passing through an object such as the human body to have a radiation image stored therein, (ii) scanning the stimulable phosphor sheet by stimulating rays which cause it to emit light in proportion to the radiation energy stored, (iii) detecting the emitted light and converting it into an electric image signal, and (iv) reproducing a visible image on a recording material such as a photographic material, or on a display device such as a cathode ray tube (CRT) by use of the electric image signal.
As disclosed, for example, in Japanese Unexamined Pat. Publication Nos. 58(1983)-67240, 58(1983)-67242 and 58(1983)-67243, it has also been proposed to embody the aforesaid radiation image recording and reproducing system so that a read-out operation for detecting the image input information of a radiation image stored in a stimulable phosphor sheet (hereinafter referred to as the preliminary read-out) is conducted in advance by use of stimulating rays having stimulation energy of a level lower than the level of the stimulation energy of stimulating rays used in a read-out operation for obtaining a visible image for viewing, particularly for diagnostic purposes (hereinafter referred to as the final read-out), and thereafter the final read-out is carried out. In the final read-out, read-out conditions and/or image processing conditions are adjusted on the basis of the image input information obtained by the preliminary read-out. This method is advantageous in that it is possible to obtain a radiation image having an improved image quality, particularly a high diagnostic efficiency and accuracy.
In X-ray image recording apparatuses, in order to control the object area exposed to X-rays, an irradiation field stop such as a collimator is often positioned near an X-ray source. In the case where the X-ray image recording apparatuses are used for recording an image of the human body for medical diagnosis, since x-rays do harm to the human body, the irradiation field stop is used for preventing portions of the human body that have no bearing on the diagnosis from being exposed to X-rays. Also, the irradiation field stop eliminates the problem that X-rays scattered by object portions outside of the portion required to be viewed, particularly for diagnostic purposes, enter the necessary portion and are recorded in a recording material, thereby deteriorating the contrast and image quality of the X-ray image of the necessary portion.
When an irradiation field stop is used, since only an object portion of which an image should be recorded is exposed to X-rays, deterioration of image quality as described above is prevented. However, when the irradiation field (X-ray exposure field) is limited to a size smaller than the size of the recording material, X-rays scattered by the object portion within the X-ray exposure field are recorded at marginal portions of the recording material outside of the X-ray exposure field. Particularly, when a recording material such as a stimulable phosphor sheet exhibiting a wide latitude is used, noise caused by X-rays thus scattered becomes far more perceptible than when a combination of the conventional film with intensifying screens is used. Noise thus generated adversely affects the marginal portions of the final visible image. Also, in the case of the aforesaid radiation image recording and reproducing system wherein preliminary read-out is conducted prior to final read-out, if noise thus generated is detected at the preliminary read-out step, there arises a very serious problem that erroneous or biased information is given for adjustment of final read-out conditions. These problems do not arise when a stimulable phosphor sheet having a size conforming to the size of the X-ray exposure field is selected and used each time image recording is conducted. However, the operations required for this are troublesome and are not practicable. Particularly, in the case of the radiation image recording and read-out system as described in EP77678A wherein stimulable phosphor sheets having equal, comparatively large sizes are incorporated in the apparatus for conducting the recording and read-out of images of various object portions, the sizes of the stimulable phosphor sheets are not changed during image recording. Therefore, in this case, too, the aforesaid problems must be solved by a different technique.